The invention relates to the field of integrated optic polarization converters, and in particular to integrated optic polarization converters based on mode evolution or structural chirality.
As the prevalence of fiber optic communication grows, the demand for more intricate processing of optical signals continues to increase. Since integrated optic devices allow for integration of many optical functions on a chip, integrated optic approaches will likely fill the demand for more intricate optical signal processing. However, in order to improve the functionality and reduce the cost per function the density of components on the chip must increase.
For a given wavelength, the confinement of a mode in a dielectric waveguide is determined by the contrast between the core and cladding indices, the higher the contrast, the tighter the confinement. An outgrowth of tighter confinement is the ability to pack waveguides closer together and guide light around sharper bends without substantial radiative loss. Since these are the two most critical parameters affecting device density, it can generally be said that the higher the index contrast the greater the device density. However, as the index contrast increases, the transverse electric (TE) and transverse magnetic (TM) modes propagating in the waveguides begin to exhibit different characteristics. While in a straight section of a square waveguide, the TE and TM modes propagate at the same rate, in a bend the TE and TM modes propagate at substantially different rates. And, when a pair of square high index contrast (HIC) guides is coupled, the TE and TM modes tend to couple at different rates. Since most integrated optic components are sensitive to both propagation velocity and guide-to-guide coupling, these effects result in polarization dependent performance, a result that is not compatible with the random polarization state emanating from the standard single mode fiber used in telecom applications.
One way to compensate these effects is to use a rectangular waveguide geometry and alter the aspect ratio of the guide to compensate for the natural difference in propagation around a bend and/or equalize the guide-to-guide coupling. However, while one or the other of these effects may be compensated in this manner for a particular device, as the index contrast increases it becomes difficult if not impossible to compensate both simultaneously in a manner that applies to all devices on the chip.
Another approach for overcoming the polarization sensitivity of HIC integrated optics is to split the random input polarization emanating from the single mode (SM) fiber with a polarizing beam splitter (PBS), couple the outputs to polarization maintaining (PM) fibers, twist one of these PM fibers by 90° degrees and couple the two fibers to separate paths on the integrated optic chip. On each of these paths identical structures are used to process the two components independently. At the output, these components are recombined by coupling to another pair of PM fibers, twisting the PM fiber of the path that had not previously been twisted and coupling both fibers to another PBS which has a SM fiber output. While such an approach, commonly referred to as a “polarization diversity” scheme, is feasible, when implemented with bulk optics it is also cumbersome. Aligning PM fibers is difficult and expensive. And, in order to preserve signal integrity the path lengths must be matched to within at least one-tenth of a bit length (i.e. ˜2 mm for 10 Gb/s signals and ˜0.5 mm for 40 Gb/s signals assuming an index of 1.5).
A better approach is to integrate the splitting function of the PBS and the rotating function of the twisted PM fiber onto the integrated optic chip. Doing so would eliminate the need to align PM fibers and path lengths could be matched easily through lithography.
Several integrated optic polarization splitters and rotators (or converters) have been proposed. However, most of the devices proposed to date rely on the coupling of a pair of waveguide modes. Devices based on coupled modes generally exhibit a wavelength sensitivity resulting from differences in the dispersion of the super-modes propagating in the structure. Further, such approaches are very sensitive to fabrication errors. Even slight changes in the waveguide geometries or separation can have a significant impact on the device performance.
A better way to form a polarization splitter or rotator is to use the principle of mode evolution. By making gradual (or adiabatic) changes to the waveguide geometry, the modes in the guide can be conditioned and the polarization states separated or rotated. Such an approach only requires that the modes not exchange power which can be assured by proper design of the waveguide and a slow evolution of the structure. Since prevention of mode coupling is a relatively loose requirement, devices based on mode evolution tend to be wavelength insensitive and fabrication tolerant. It has been proposed and demonstrated that a polarization splitter based on mode evolution can be formed, however, this approach has the disadvantage of requiring multiple waveguide materials. And, to date no polarization converters have been proposed which utilize the principle of mode evolution.
Generally, it is the object of the invention to convert a TM input polarization to a TE output polarization or vice versa with an integrated optic device based on the principle of mode evolution or structural chirality.
It is yet another object of the invention that the device be wavelength insensitive, tolerant to fabrication errors, and require only a single material system to construct.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description and accompanying figures.